OCR A Jun 2014 Paper 2 Q1 (with explained solutions) |

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OCR A Jun 2014 Paper 2 Q1

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Answer all the questions. This question is about cyclic organic compounds. The table shows some information about cycloalkanes. Cycloalkane Skeletal formula Boiling point / C Cyclopropane Cyclopentane Cyclohexane 33 49 81 (a) These cycloalkanes are members of the same homologous series and have the same general formula. (i) What is meant by the term homologous series?[2] (ii) State the general formula for these cycloalkanes.[1] (iii) Explain the increase in boiling points of the cycloalkanes shown in the table.[2]OCR 2014 Turn over<br />
(b) The CCC bond angles in cyclohexane are 109.5. State and explain the shape around each carbon atom in cyclohexane. shapeexplanation[2] (c) In the absence of ultraviolet radiation, cyclopropane undergoes an addition reaction with bromine. Suggest the structure of the organic product formed in this reaction. [1] (d) Petrol contains both cyclohexane, C6H12, and hexane. Cyclohexane can be prepared from hexane. (i) Construct the equation for this reaction.[1] (ii) Suggest one advantage of adding cyclohexane to hexane in petrol.[1] (e) Cyclobutane is another cycloalkane. There are several unsaturated isomers of cyclobutane that are alkenes. Two of these isomers are stereoisomers. (i) Explain what is meant by the term stereoisomers.[1] (ii) Draw these two stereoisomers.OCR 2014 [2]<br />
(f) In the presence of ultraviolet radiation, cyclohexane reacts with bromine. A mixture of cyclic organic compounds is formed, including C6H11Br. (i) Complete the table below to show the mechanism of the reaction between bromine and cyclohexane to form C6H11Br. Include all possible termination steps in your answer. Step Initiation Propagation EquationTermination(ii) The initiation step involves homolytic fission. Explain why the initiation step is an example of homolytic fission. [5][1] (g) The reaction between cyclohexane and bromine in (f) also forms C6H10Br2. (i) Write an equation, using molecular formulae, for the reaction of cyclohexane and bromine in the presence of ultraviolet radiation to form C6H10Br2.[1] (ii) Name one of the structural isomers of C6H10Br2 formed in the reaction between cyclohexane and bromine.[1]OCR 2014 [Total: 21] Turn over<br />

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F322 Question 1 (i) (a) Answer (series of compounds with the) same functional group OR same/similar chemical properties OR same/similar chemical reactions (cid:57) each successive/subsequent member differing by CH2 (cid:57) (ii) CnH2n (cid:57) (iii) More carbons (in ring) OR more (surface area of) contact AND more van der Waals forces OR stronger van der Waals forces (cid:57) More energy needed to break the intermolecular forces (cid:57) Mark Scheme June 2014 Mark 1 2 Guidance IGNORE references to physical properties IGNORE has same general formula (in question) DO NOT ALLOW have the same empirical formula OR have the same molecular formula Both answers need to be comparisons ALLOW ORA throughout ALLOW has more electrons OR larger (carbon) ring OR higher molecular mass IGNORE bigger molecule IGNORE chain instead of ring DO NOT ALLOW more contact between atoms ALLOW VDW for van der Waals More intermolecular forces is not sufficient ALLOW it is harder to overcome the intermolecular forces ALLOW intermolecular bonds / van der Waals bonds ALLOW more energy is needed to separate molecules IGNORE more energy is needed to break bonds<br />
F322 Question (b) Answer tetrahedral (cid:57) four bonding pairs repel OR four bonds repel (cid:57) Mark Scheme Mark (c) (cid:57) Br Br (d) (i) C6H14 (cid:198) C6H12 + H2 (cid:57) June 2014 Guidance Mark each point independently IGNORE surrounded by four atoms IGNORE four areas of electron charge repel IGNORE four electron pairs repel (one could be lp) DO NOT ALLOW atoms repel ALLOW correct structural OR displayed OR skeletal formula OR mixture of the above (as long as unambiguous) ALLOW structure of 1,2-isomer Br Br IGNORE molecular formula DO NOT ALLOW, structure of 1,1-isomer OR 2,2-isomer Br Br ALLOW correct structural OR displayed OR skeletal formula OR mixture of the above (as long as unambiguous) ALLOW any correct multiple IGNORE state symbols<br />
F322 Question (ii) Cyclohexane will burn more efficiently (cid:57) Answer Mark Scheme Mark (e) (i) (ii) (Compounds with the) same structural formula but a different arrangement (of atoms) in space (cid:57) CH3 H3C (cid:57) (cid:57) H3C CH3 June 2014 Guidance KEY IDEA IS COMBUSTION OR BURNING Assume it refers to cyclohexane ALLOW ORA for hexane ALLOW cyclohexane allows smoother burning OR promotes more efficient combustion OR increases octane number OR reduces knocking OR less likely to produce pre-ignition OR burns better OR easier to burn OR combusts more easily OR improves combustion OR burns more cleanly DO NOT ALLOW cyclohexane ignites more easily IGNORE cyclohexane increase volatility of fuel IGNORE reference to boiling points IGNORE cyclohexane gives a better fuel ALLOW different spatial arrangement of atoms. DO NOT ALLOW different displayed formula. ALLOW displayed OR skeletal formula OR mixture of the above. ALLOW structures in either order IGNORE molecular formula IGNORE structural formula IGNORE names IGNORE E/Z and cis/trans labels ALLOW 1 mark for a pair of E/Z isomers of an incorrect hydrocarbon structure with four C atoms e.g. C, or CH or CH2 instead of CH3 groups.<br />
F322 Question (i) (f) Mark Scheme Mark Step Initiation (1 mark) Propagation (2 marks) Termination (2 marks) Answer Equation Br2 (cid:198) 2Br (cid:57) C6H12 + Br (cid:198) C6H11 + HBr (cid:57) C6H11 + Br2 (cid:198) C6H11Br + Br (cid:57) C6H11 + Br (cid:198) C6H11Br C6H11+ C6H11(cid:198) C12H22 Br + Br (cid:198) Br2 Two correct (cid:57) All three correct (cid:57)(cid:57) (ii) The breaking of a (Br-Br) bond AND forms (two) radicals OR the breaking of a (Br-Br) bond AND one electron (from the bond pair) goes to each atom/bromine (cid:57) (g) (i) C6H12 + 2Br2 (cid:198) C6H10Br2 + 2HBr (cid:57) 1,1-dibromocyclohexane OR 1,2-dibromocyclohexane OR 1,3-dibromocyclohexane OR 1,4-dibromocyclohexane (cid:57) (ii) June 2014 Guidance IGNORE state symbols IGNORE dots If an incorrect hydrocarbon with six C atoms is used: DO NOT ALLOW any marks for the propagation steps but ALLOW ECF for termination steps (i.e. 3 max) ALLOW the breaking of a covalent bond ALLOW the splitting of the bond in bromine ALLOW the breaking of a covalent bond where each atom keeps one of the bonding electrons IGNORE particle for atom ALLOW one electron goes to each product / species DO NOT ALLOW molecule or compound for atom IGNORE homolytic fission equations ALLOW molecular formula only. Locant numbers MUST lowest possible e.g. DO NOT ALLOW 2,4-dibromocyclohexane etc. IGNORE structures Total 21<br />

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